The maximum heart rate can only be reliably determined through direct exertion to failure, such as during a VO2 max or stress test.

“I think the only way to really know it is to actually push yourself in um kind of sub maximal efforts until you get to maximum heart rate so this will be typically be seen um if a person does a VO2 max test um and certainly will be seen if a person does a stress test if they're pushed to failure”